The present invention is of particular significance in view of the increasing use of digital transmission and switching in voice and data telecommunication networks and the need for accurate information transmission through these networks. Heretofore, digital transmission systems have relied on "pulse stuffing" techniques to ensure accurate transmission of information in a digital multiplex hierarchy. With this approach, a higher level of the hierarchy designed to multiplex n lower levels signal together, each of rate R, will operate at a rate somewhat greater than Rn. Additional "stuffing bits" are added to each lower level multiplex group to bring the individual group rates up to the rate necessary for the higher level multiplex. These stuffing bits are removed when the signals are demultiplexed.
This approach is not appropriate for a switched network. Digital switching requires equal rates for individual channels. This is necessary to allow information from a channel of one multiplex stream to be switched to a different channel of a different multiplex stream without errors caused by timing difference. If the first channel were faster than the second, all of the incoming bits could not be accommodated in the outgoing channel. If the second channel were at a higher rate, there would not be sufficient incoming information bits to fill the outgoing channel. Both of these situations would cause errors. It should also be noted that the same situation prevails in a purely multiplex system that has a requirement to drop and insert digital channels.
Various approaches to network synchronization have been proposed to equalize clocks throughout a large network. Master/slave arrangements are possible wherein, for example, a relatively expensive atomic standard is used as a clock source and other clocks are slaved to this master. Mutually synchronized clock systems have also been studied with individual clocks arranged in a manner that attempts to achieve an average network frequency based on information from each clock. The use of highly stable, independent clocks is yet another possibility. Even with these measures, it is not possible to completely control clock signals and slip errors will eventually occur.
In a network with highly stable clocks, the occurence of such slip errors will be infrequent, occurring once in every several hours on each multiplex link. The infrequent occurence of these slip errors makes them tolerable. There are situations, however, that give rise to higher slip error rates that do represent a potentially serious system impairment. Consider, for example, transmission from a mobile station over a multiplex radio transmission link with high doppler frequency offset. With relative velocities of 1000 km/h, the doppler shift will be approximately one part in 10**6. With a doppler shift of one part in 10**6, relative timing will drift 125 microseconds in a period of (125)(10**-6)(10**6)=125 seconds. An interval of 125 microseconds corresponds to the time for one eight bit pulse code modulated voice sample at 64 kilo bits per second. Thus with an eight bit digital switch and appropriate buffering, a timing slip would occur every 125 seconds. This slip rate would be unacceptable to users with encrypted voice traffic wherein loss of crypto-sync would result. The present invention is particularly useful, then, for timing adjustment of multiplex signal streams from high-speed mobile terminals. These terminals may have additional equipment for voice signal encryption. The circuitry disclosed in the present invention ensures proper operation of this equipment by removing slip errors.